Thanks serfty, but I don't quite see what you're getting at. Although musical instruments are listed amongst sporting items for purposes of length, they don't seem to say that musical instruments = sporting equipment for purposes of the 50% excess baggage charge.
I suppose it is possible they could say a guitar is too big b/c it is longer than 100cm, but I think that is unlikely - a guitar is still a lot smaller than a bicycle!

Sorry if I'm not making much sense, I'm not really awake yet. And thanks again.